Vacuum oven



July 12, 1966 F. A. POTENZO ETAL 3,260,783

VACUUM OVEN Original Filed Feb. 17, 1961 K36 INVENTORS FRANK A. POTENZOATTORNEYS United States Patent 3,260,783 VACUUM OVEN Frank A. Potenzoand Raymond L. Shearer, Kennebunlr,

Maine, assignorsto The'Baker Company, Inc., Biddeford, Maine, acorporation of Maine Original application Feb. 17, 1961, Ser. No.90,090, now Patent No. 3,160,404, dated Dec. 8, 1964. Divided and thisapplicationOct. 23, 1964, Ser'. No. 411,666

2 Claims. (Cl. 13--31)v This is a division of copending applicationSerial No. 90.090,fil d'Fe-bruaryl7, 1961, now'Patent No. 3,160,404,dated'December 8, 1964.

This invention relates to industrial ovens, or moreparticularly, to'a'novetimprov'ement in what are generally referred to'aslvacuum ovens.

lncertain fields, and. particularly'the fieldof semiconductormanufacture, there has developed a need for relatively smallovens whichcan be arranged in conjunction withali'ne' of sealed work spaces whichare referredto as'dry boxes. A controlled environment is maintained inthe dry boxes. Gas lock installations between the dry boxes allow theparts to be transferred from one station to another without dangero-fftransferring contaminants. The dry boxes are fitted with hand holeswhich accommodate surgicalr'ubbe-r gloves or the like whereby, a workmancan handl-epartswithin the dry box from externally of thedrybox'withou't destroying the integrity of the box.

Vacuumovens may be placed in the line of'dry boxes so thatwork can enteran oven from one'dry box and be removedifrorn the oven into another drybox. In some instances, a vacuum'oven'may bemounted at the end of a lineso th'at parts may be placedin the oven through one door therein andthenremoved from another'door communicating with one of the dry boxes. Thevacuum oven derives its namefrorn the factthatthe oven is evacuated to arelatively'lo'w pressure during the heattre'at-ment of the partsiin theoven. The ovens are usually fitted with sh'elves'arranged vertically intheoven and on which parts may be placed forheat treatment.

One of the problems which has. arisen in connection with vacuum ovens isthat of heat'distribution in the oven. Forexample, inthe manufacture oftransistors, it is import-antthat-th'e temperature at which the partsare treated is maintained very closely. With many small parts spreadover aplurality, of vertically arranged shelves, it is importantthat'the heat distribution within the oven,,both in a vertical "and inahorizontal direction, be substantially uniformso as to assure that eachof the parts is exposed to the same desired temperature. Another problemwhich has arisen in connection with the-vacuum oven is concerned withthe sealing around the doors ofthe oven. Because it'is more resistant tohigh temperature, silicon rubber is a preferred material for use as adoor seal. However, when thesilicon mhber is exposed tohigh temperatureand 'at'the same time, is exposed to very low pressures,

the volatiles in the silicon rubber tend to boil off readily. Thesevolatiles may contaminate the parts in the oven. Further, the rubberquickly, becomes hard and brittle and deteriorates into the conditionwhere-it no longer performs the desired function.

It isacc-ordingly an objectof this invent-ion to provide a vacuum ovenof the type described which will provide ice The invention accordinglyconsists in the features of construction, combination of elements andarrangement of parts which will be exemplified in the constructionhereafter set forth and the scope of the application of which will beindicated in the appended claims.

In the drawings:

FIG. 1 is a cross sectional view of a vacuum oven incorporating'thepresent invention; and

FIG. 2 is an enlarged fragmentary cross sectional view of theportion oftheoven of FIG. 1 enclosed by brokenlines 2.

With reference to the drawings, a vacuum oven ofatype with which thisinvention is concerned generally comprises a stainless steel mufile 10which, in the specific embodiment shown, is an elongated hollow memberhaving a generally rectangular cross section and being open at itsopposite ends. The mufile. is contained within a housing which comprisesa pair of metal flanges 12 (only one of which is shown). which arewelded to the nrufiie adjacent the opposite ends thereof. The flanges 12extend completely around the motile andextend outwardly at right anglestherefrom. A sheet metal shell 14 is suitably mounted .on the outer endsof the flanges 12 so as to surround the mufiie in spaced relation andcomplete the housing for the muffle. The oven is adapted to be mountedinan aperture in a wall or frame 16. To facilitate mounting of the oven, aplurality of studs are welded to the flanges or end plates 12. As shownin FIG. 1, the studs extend through a gasket lfidisposed between the endplates 12 and'wall16, and a plurality of nutsretain the parts inassembly. This manner of mounting the oven is, of course, only shown byway of example, and other suitablemeans could be used.

The oven further comprises apair of hinged doors 2t and 22 which formendwalls for the'opposite ends of the back fill pipe 26 also extendsfrom the bottom wall of l themufileand through the shell14. The backfill pipe is intended to be valved in order to provide means forbreaking the vacuum in the oven and permit the pressure within themutile to return to atmospheric pressure prior to opening the doors. Aplurality of vertically arranged shelves 27 are provided within themuffle to accommodate parts to be treated within the oven. The shelvesare fabricated from a ferrous metal, preferably stainless steel. Theshelves are perforated to facilitate heat transfer vertically within themuffle and are preferably slidably mounted for ready removal from themufile.

In order to provide a heat source for'the oven, a plurality of'electrical resistance heaters 28" are arranged aroundthe top, bottom andside Walls of the muffle. In the specific embodiment, these heaters areofa type generally referred to as tubular heaters. More specifically,

as shown in FIG. 1, a plurality of such heaters are arranged fle. Thesandwiching of the heaters between the plates 30' and 32facilitatesmounting of the heaters on the mufiie. Further, as can beseen from FIG. 1, each plate assembly is ofrelatively large planar areaand covers the majority of the particular wall of the mufile. It ispreferred that the plates have a relatively high thermal conductivity sothat the heat provided by the heaters will be substantially uniformlydistributed over the areas covered by the respective heater plateassembly. In this manner, the tendency of the heaters to provide hotspots within the mufile in the immediate area of the heaters will besubstantially eliminated, and there will tend to be uniform heatdistribution within the muffle.

As noted above, one of the problems in vacuum ovens of the type shown isthat of providing optimum uniform heat distribution within the mufile.In order to provide more uniform heat distribution both vertically andhorizontally within the oven, the inner walls of the muflle, the endwalls formed by the doors 20, 22, the shelves 27 and the inside of thevacuum draw pipe 24 and back fill pipe 26 are Black Oxide coated. Thus,all of the surfaces exposed to or seeing the inside of the muflle areBlack Oxide coated. In accordance with the invention, the treatmentprovided serves to increase the resistance to oxidizing of the surfacestreated, While at the same time provides the inner surfaces of themufile with a much higher thermal emissivity than would be provided bythe usually highly reflective surface provided by stainless steel. Theproblem of heat distribution in an oven of the type described iscomplicated by the fact that the heat treatment is conducted under apartial vacuum, for example at pressures from l l to 1X10" millimetersof mercury. At this low pressure, heat transfer within the mufile issubstantially entirely by means of radiation rather than by convection.The Black Oxide coating provided in accordance with this invention willincrease the radiation heat transfer from the mufi le walls to improvethe efiiciency of the oven. Also, the particular treatment utilizedprovides a coloration and finish which is quite uniform so as to providethat the thermal emissivity of the inner surfaces of the mufile will beuniform and there will be no reflective portions which could cause hotspots. The Black Oxide treatment specifically contemplated involvesheating of the mufiie at a relatively high temperature in an alkalinebath. Such treatments are normally of a proprietary nature. A specificone of such treatments contemplated is provided by the Rust Proofing andMetal Finishing Corporation of Cambridge, Massachusetts, under the nameof Black Oxide No. 160. It will be understood that where the term BlackOxide is used herein, the term refers to a treatment such as that justidentified.

As noted above, another problem in the operation of vacuum ovens is therapid deterioration of the door seals because of the exposure of theseals to relatively high temperatures and low pressures. Theseenvironmental conditions tend to cause any volatiles in the sealmaterial to boil off leaving the seal in a hard brittle condition whichwill seriously reduce its utility for the purpose intended. Withreference to FIGS. 1 and 2, the door sealing arrangement is identicalwith respect to the doors 20 and 22. Accordingly, in the interest ofbrevity, only the details of construction of the door 20 and its sealingarrangement have been shown and will be described. The door 20 comprisesa sheet metal shell 36 of rectangular cup-like configuration. A plate 38is disposed within the door shell 36 and is fixed to the upstanding sidewalls of the door shell intermediate the ends of the walls so that theplate 38 extends parallel to the general plane of the door in spacedrelation generally midway between the ends of the side walls of the doorshell 36. The door further includes a stainless steel pressure plate 40forming an inner wall for the door and an end wall for the muffle. Thepressure plate 40 extends parallel to the general plane of the door andis mounted for movement toward and away from the plate 38. Morespecifically, a plurality of pins 42 are mounted on the pressure plateand extend at right angles thereto and inwardly of the door. The pins 42extend through apertures in the plate 38 with the bordering edgeportions of the apertures supporting the pins for sliding movement. Aplurality of coil springs 44 are arranged over the pins 42 and engagedat their opposite ends between the pressure plate 40 and plate 38 forthe purpose of urging the pressure plate away from the plate 38. Thefree ends of the pins 42 are provided with radially extending stops 46to limit movement of pins, and thus the pressure plate, away from theplate 38. When the door is open, the springs 44 will urge the pressureplate outwardly of the door until the stops 46 engage the plate 38. Whenthe door is closed, the pressure plate 40 engaged against a rectangularcross sectioned silicon rubber seal 48 arranged around the outside ofthe mufile. When the door is securely locked in the closed position, theseal 48- engages the pressure plate 40 and will cause the pressure plate48 to be moved inwardly of the door against the force of the springs,whereby the pressure plate will be maintained in firm engagement withthe seal 48.

The seal 48 extends entirely around the periphery of the muffle and, ascan be seen in FIG. 2, the seal projects beyond the end of the muffle.In order to provide cooling for the seal 48, a cooling ring 50 ismounted on the end wall 12 of the oven housing. The cooling ring 50 isan elongated, rectangular cross sectional, hollow member extendingcompletely around the seal 48 in contact with the side thereof oppositethe muffle. A plurality of fittings 52 are provided on the cooling ring.These fittings extend through apertures in the end wall 12 of the ovenhousing and are adapted to be connected to a source of cooling liquid,such at water, as well as to a drain, whereby the cooling liquid may becontinuously circulating through the cooling ring. As will be seen inFIG. 2, the wall of the cooling ring next adjacent the door is spacedfrom the pressure plate 40 when the door is closed. The flat engagedsurfaces of the seal and cooling ring assure good heat transfer betweenthese members inasmuch as these surfaces conform to each other and theover-lying surface of the ring is generally coextensive with theunderlying surface of the seal. The cooling of the seal in the mannerprovided tends to alleviate the problems of the boiling off of thevolatiles in the seal when the oven is operated at relatively hightemperatures, for example 250 C. and relatively low pressures specifiedabove. However, as noted above, the exposure of the seal to hightemperature is only a portion of the problem; the other being theexposure of the seal to low pressures. Also, as noted above, the seal 48projects beyond the end of the muifie so that, when the door is closed,the pressure plate 40 will be spaced from. the end of the mufile. Thisassures that firm sealing will be provided between the pressure plateand silicon seal, and that there will be no metal to metal contactbetween the pressure plate and the mufile. However, the spacing of theends of the muflie from the pressure plate is particularly importantwith regard to another aspect of the invention.

More specifically, the end of the muflie is spaced from the pressureplate a distance selected to provide what will be referred to as anoptical seal between the interior of the mufile and the silicon seal,whereby the lowest pressure to which the seal 48 will be exposed will begreater than the lowest pressure achieved within the interior of themufile. The term optical seal connotes a seal whereby the silicon seal48 does not see the pressure within the muflie. In the embodiment ofFIG. 2, the space 54 between the muffle and pressure plate isapproximately &2 of an inch, which dimension is quite small as comparedto the overall dimensions of the oven. In the specific embodiment, theoven is approximately 10 /2 inches in its cross sectional diameter andapproximately 14 inches long. It will be apparent from FIG. 2 that theonly portion of the seal directly exposed to the interior of the mufileis that portion which projects beyond the ends of the muffle. When theoven is evacuated, the air at the outer end portions of the gap or space54 can be drawn out only by passing through the very narrow passageprovided by the space 54 and into the interior of the muffle. As the airis drawn from the muflle, there will be no flow of air through thepassage 54 which will readily sweep the molecules of air from the.passage 54. This is particularly true as the pressure within the muffleis substantially reduced. Rather, the molecules of air in the passage 54will tend to move between the opposing surfaces of the end of the muffleand pressure plate 40 substantially at right angles to these surfacesinstead of moving parallel to these surfaces. This action materiallyretards the evacuation of the space 54 with the result that a higherpressure will be present within the passage 54 than will be presentwithin the main body of the muffle. This will be particularly true inthe area of the passage immediately adjacent the seal 48. It can be seenthat the molecules of air in the passage 54, and particularly at the endthereof adjacent the seal 48, follows what amounts to a tortuous pathinto the interior of the muflle. Admittedly, their path is not astortuous as would be the case if the passage 54 were a labyrinthpassage. However, the configuration shown has been found to besufficient for the purpose intended. It is to be understood, however,that a more complex optical seal is within the scope of this invention.Admittedly, there Will be a tendency of the air molecules within thepassage 54 to diffuse into the main body of the muffle until thepressure within the passage 54 is equal to the pressure within themuffle. However, such diffusion will require a substantial length oftime to occur. In practice, the oven will normally be opened every threeor four hours, and in this short length of time, the advantages gainedby the optical seal will not be lost to any substantial extent bydiffusion.

Accordingly, it can be seen that by the door sealing arrangement of thisinvention the silicon rubber seal is both exposed to temperaturesconsiderably less than that within the muflle and is exposed topressures substantially greater than the pressure within the muflle. Bythis means, the service life of the seal has been materially improved.The specific arrangement of the cooling ring and rubber seal results inthe cooling ring being disposed remotely from the muffle so as not tocause cooling of the ends of the muffle with an attendant impairment ofthe uniformity of heating in this area of the muffle. Also, the coolingring is not in contact with the pressure plate so as to precludeundesired cooling of the end wall of the mufile formed by the pressureplate.

Inasmuch as changes could be made in the above construction andapparently different embodiments of this invention could be made withoutdeparting from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the language in the following claims isintended to cover all the generic and specific features of the inventionherein described and all statements of the scope of the invention which,as a matter of language, might be said to fall therebetween.

Having thus described our invention, we claim:

1. In a vacuum oven, a ferrous metal muffle having side, top and bottomwalls and having end walls, electrical resistance heating means arrangedin intimate heat trans fer relation over the majority of the area of theouter surface of each of said side, top and bottom walls of the muffle,at least one of said end walls being formed. by a door which closes oneend of the muflle and which includes a ferrous metal plate-like memberextending across said one end of the muffle in sealed relationthereto, aBlack Oxide coating on the inner surfaces of all of the aforementionedwalls of the muflle, said Black Oxide coating being evenly distributedover the entire area of the muffle walls and materially improving thethermal emissivity of said top, bottom, and side walls and materiallyimproving the uniformity of heat distribution both vertically andhorizontally through the interior of the muffle when the over isoperated at a vacuum, and means for drawing a vacuum on the interior ofthe muffle.

2. In a vacuum oven a stainless steel muffle having side, top and bottomwalls and having end walls, electrical resistance heating means arrangedin intimate heat transfer relation over the majority of the area of theouter surface of each of said side, top and bottom walls of the muffle,at least one of said end walls being formed by a door which closes oneend of the muflle and which includes a stainless steel plate extendingacross said' one end of the muffle in sealed relation thereto, aplurality of stainless steel shelves arranged vertically within theoven, a Black Oxide coating on the inner surfaces of all of theaforementioned walls of the muffle and on said shelves, said Black Oxidecoating being evenly distributed over the entire area of the muflleWalls. and shelves to materially improve the thermal emissivity of saidtop, bottom and side walls and materially improve the uniformity of heatdistribution both vertically and horizontally through the interior ofthe muflle when the oven is operated at the vacuum, and means fordrawing a vacuum on the interior of the muffle.

References Cited by the Examiner UNITED STATES PATENTS 1,127,021 2/1915Klett 219-407 2,035,757 3/1936 Ottenstein 219-407 2,078,840 4/1937 Dorlet al. 219-406 2,085,450 6/1937 Rohn 2257 2,156,845 5/1939 Gentile 34732,251,380 8/1941 Warrender 13--3l 3,028,476 4/1962 Hug 219201 X3,118,042 1/1964 Parker 219406 OTHER REFERENCES Westinghouse Engineer,Control of Radiant Heat by Surface Finish, July, 1954, pp. 147-151.

RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner.

1. IN A VACCUM OVEN, A FERROUS METAL MUFFLE HAVING SIDE, TOP AND BOTTOMWALLS AND HAVING END WALLS, ELECTRICAL RESITANCE HEATING MEANS ARRANGEDIN INTIMATE HEAT TRANSFER RELATION OVER THE MAJORITY OF THE AREA OF THEOUTER SURFACE OF EACH OF SAID SIDE, TOP AND BOTTOM WALLS OF THE MUFFLE,AT LEAST ONE OF SAID END WALLS BEING FORMED BY A DOOR WHICH CLOSES ONEEND OF THE MUFFLE AND WHICH INCLUDES A FERROUS METAL PLATE-LIKE MEMBEREXTENDING ACROSS SAID ONE END OF THE MUFFLE IN SEALED RELATION THERETO,A BLACK OXIDE COATING ON THE INNER SURFACES OF ALL OF THE AFOREMENTIONEDWALLS OF THE MUFFLE, SAID BLACK OXIDE COATING BEING EVENLY DISTRIBUTEDOVER THE ENTIRE AREA OF THE MUFFLE WALLS AND MATERIALLY IMPROVING THETHERMAL EMISSIVITY OF SAID TOP, BOTTOM AND SIDE WALLS AND MATERIALLYIMPROVING THE UNIFORMITY OF HEAT DISTRIBUTION BOTH VERTICALLY ANDHORIZONTALLY THROUGH THE INTERIOR OF THE MUFFLE WHEN THE OVER ISOPERATED AT A VACCUM, AND MEANS FOR DRAWING A VACCUM ON THE INTERIOR OFTHE MUFFLE.